There are a variety of electrical systems, which require one or more sources of high voltage AC power for controlling the operation of a system application device. As a non-limiting example, a liquid crystal display (LCD), such as that employed in desktop and laptop computers, or in larger display applications such as large-scale television screens, requires an associated set of high AC voltage-driven cold cathode fluorescent lamps (CCFLs) mounted directly behind it for backlighting purposes. Indeed, large LCD panels require relatively large numbers (e.g., on the order of ten to forty) of such lamps for uniform backlighting.
In order to achieve uniform brightness of all the lamps, several inverters (voltage-controlled switching devices and associated output transformers coupled to the lamps) are required, with each inverter generating a high voltage sine wave that typically drives from one to five parallel-connected lamps. In such an architecture, the switching frequencies of all the inverters must be mutually synchronized, in order to avoid uncontrolled electromagnetic interference at sum and difference frequency values of the various switching frequencies. This is customarily accomplished by distributing a synchronizing signal among the inverters to set the switching frequency.
Adjusting the brightness of (or dimming) a CCFL may be effected by means of a PWM dimming signal, which controllably switches the lamp drive voltage and current off for brief periods of time. In accordance with this technique, the CCFL is turned ON and OFF for relatively short periods of time (e.g., from 0.1 to 5 msec. each), with the brightness of the lamp being proportional to the PWM duty cycle. This methodology is customarily carried out by applying a separate PWM dimming signal to each inverter. In addition to PWM dimming, analog dimming may be used to increase the range of dimming provided by PWM dimming. In a typical application, an analog control signal is supplied to each inverter in order to set the current flowing through the lamp. The brightness of the lamp output is adjusted by controllably increasing or decreasing the amplitude of the analog signal.